A123998: No primes except 3?

[Max A.]

Note that if n = 1 mod 3 then 2n+1 is not prime (except n=1);
and if n = 2 mod 3 then 4n+1 is not prime.
Therefore, n must be a multiple of 3, except n=1.

Max

If n>1 and both 2n+1, 4n+1 are primes

On 11/2/06, zak seidov <zakseidov at yahoo.com> wrote:
> Why are there no prime numbers except of 3,
> in this sequence?
> Zak
>
> A123998
> ={1,3,9,15,18,39,48,69,78,99,105,114,135,153,165,168,183,189,219,249,273,
>
> 288,300,303,309,330,345,363,405,414,438,468,483,498,504,534,585,618,
>  639,648,699,714,729,765,804,813,828,879,933};
> (* %N A123998 Numbers n for which 2n+1 and 4n+1 are
> primes. *)
>
>
>
>
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